The manufacture of devices in microelectronics or microtechnology requires, in the more advanced solutions, the production of air gaps.
To produce these air gaps, one of the current approaches consists in degrading a sacrificial material, typically silicon oxide, by means of a chemical attacking agent, for example hydrofluoric acid, which must cross a membrane film to reach this material.
Besides being capable of allowing the passage of the chemical attacking agent, this membrane must satisfy very precise specifications, namely, it must:
be itself resistant to the chemical attacking agent,
be compatible with the various processes and treatments used for producing the structure into which it is incorporated (metallizations, mechanochemical polishing, thermal annealing, and the like) and, in particular, be stable at temperatures that may be up to 400° C.,
have satisfactory mechanical properties since it forms part of the framework of the structure, and
have a low dielectric constant, i.e. not more than 4.0, in the case of an interconnect structure for an integrated circuit.
The permeable membranes currently used are generally polymers of polyphenylene type that are deposited via the spin-coating technique. These polymers have a high coefficient of thermal expansion and low mechanical properties. Thus, their expansion during the thermal annealing steps generates stresses in the structures that can result in delamination at the interfaces.
Furthermore, the spin-coating technique is not a favoured technique of the semiconductor industry in the case of materials that are intended to remain inside devices, chemical or physical vapour deposition techniques being largely preferred.